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Keywords:

  • affective disorder;
  • epidemiology;
  • old age psychiatry;
  • public health;
  • stress

Abstract

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. ACKNOWLEDGMENTS
  7. REFERENCES

Aim:  The purpose of the present study was to clarify the relationship between late-life depression and daily life stress in a representative sample of 10 969 Japanese subjects.

Methods:  Data on 10 969 adults aged ≥50 who participated in the Active Survey of Health and Welfare in 2000, were analyzed. The self-administered questionnaire included items on 21 reasons for life stressors and the magnitude of stress, as well as the Japanese version of the Center for Epidemiologic Studies Depression Scale (CES-D). The relationship between the incidence of life stressors and mild–moderate (D16) and severe (D26) depressive symptoms was examined using logistic regression analysis.

Results:  A total of 21.9% of subjects had D16 symptoms, and 9.3% had D26 symptoms. Further, increased age and being female were associated with more severe depressive state. Logistic regression analysis indicated that the strongest relationship between both the incidence of D16 and D26 symptoms and life stressors stemmed from ‘having no one to talk to’ (odds ratio = 3.3 and 5.0, respectively). Late-life depression was also associated with ‘loss of purpose in life’, ‘separation/divorce’, ‘having nothing to do’, ‘health/illness/care of self’, and ‘debt’.

Conclusion:  There is a relationship between late-life depression and diminished social relationships, experiences involving loss of purpose in life or human relationships, and health problems in the Japanese general population.

WITH A 12-MONTH prevalence rate of 3–5%1,2 and a lifetime prevalence rate of 3–20%,3 depression (major depression) is a highly prevalent and serious disorder with significant clinical and socioeconomic ramifications. Based on the disability-adjusted life year (DALY), a measure developed by the World Health Organization (WHO), depression is projected by the year 2020 to become the second leading burdensome disease following coronary heart disease, imposing a tremendous health burden upon people. Patients with depression experience marked impairments in life functioning and well-being, and are reported to exhibit a reduction in social functioning at a level equivalent to, or more significant than, those living with chronic physical illness such as cardiopulmonary disease, arthritis, hypertension, and diabetes.4,5

Of the general population aged ≥65, approximately 10–15% are estimated to be depressed and 1–3% are estimated to have major depression.6,7 Older adults with depression have poor clinical outcomes. In a meta-analysis of 24-month clinical outcomes among the elderly with depression, only 33% were healthy, while 33% remained depressed and 21% had died.8

Depression is the most serious psychiatric disorder in late life that is associated with suicide.9 Results from WHO research investigating the types of psychiatric disorders in suicide victims at the time of their death using techniques such as psychological autopsy indicate that approximately 30% of suicide victims had a mood disorder.10 The total number of suicides in Japan, which is known for its high suicide rate, exceeded 30 000 in 2007; 36.6% and 21.3% of the suicide victims were people aged ≥60 and those in their 50s, respectively. Therefore, nearly 60% of all suicides were committed by individuals in late life, that is, people aged ≥50 (42% of the population at the time). Thus, improvement of mental health among people in late life is considered to be medically urgent in order to prevent an increase in suicides in a progressively aging society.

The entire clinical course of a psychiatric disorder – from onset to recovery – is affected by biological, psychosocial, and environmental factors in a complex manner. Although psychiatric symptoms are largely determined by biological factors, their clinical outcomes are exacerbated by psychosocial stress.11 Risk factors for depression identified in research include neurotransmitter abnormalities, sleep disorders, hormone imbalance, substance use, premorbid personality, and stressful life events.12–14 Stressors that may trigger depression, such as decreased physical and mental functioning due to aging, high prevalence of physical illness, hospitalization, and changes in living environment (e.g. retirement, living alone), are especially salient in late life. Risk factors for the incidence and recurrence of late-life depression have been found to include impairments due to physical illness, fatigue of caregiving, and psychosocial stress such as bereavement and social isolation.15 Although these insights suggest that psychological stress plays an important role in late-life depression, this has not yet been investigated in a large-scale study using a representative sample of the Japanese general population. The aim of the present study was therefore to clarify the relationship between the incidence of psychosocial stress in daily life (life stressors) and depressive symptoms among more than 10 000 late-life adults selected from 300 communities in Japan.

METHODS

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. ACKNOWLEDGMENTS
  7. REFERENCES

Subjects

The present study was conducted as part of the Active Survey of Health and Welfare (ASHW), a nationwide survey on sleep, mood, stress, and related coping behaviors conducted by the Japanese Ministry of Health, Labour and Welfare in June 2000. The purpose of the ASHW, which was conducted in 1996, 1997, 1999, and 2000, was to provide data to aid the Government's health and welfare policy making. To ensure that the sample was representative of the general population, survey participants were selected among individuals aged ≥12 living in 300 communities in Japan. These communities were selected from 881 851 precincts identified in the 2000 Census using a stratified sampling design. In each community, a part-time investigator employed by the local public health center delivered the self-administered questionnaire to the participants and collected the completed questionnaires a few days later. Oral informed consent was obtained from all subjects.

Table 1 lists the age distribution and male : female ratio of the final study sample with corresponding statistics calculated from the Census data from the same year.

Table 1.  Subject characteristics (n = 10 969) vs 2000 Census data
Age group (years)Present studyCensus (2000)(thousand)
Total (%)Female (%)Male (%)M/FTotal (%)Female (%)Male (%)M/F
50–595 036 (45.9)2583 (44.3)2453 (47.7)0.9519 176 (39.2)9 676 (36.6)9 500 (42.3)0.98
60–693 436 (31.3)1745 (30.0)1691 (32.9)0.9714 841 (30.3)7 735 (29.2)7 107 (31.6)0.92
70–791 802 (16.4)1003 (17.2)799 (15.5)0.8010 051 (20.5)5 755 (21.8)4 296 (19.1)0.75
80–695 (6.3)495 (8.5)200 (3.9)0.404 848 (9.9)3 279 (12.4)1 569 (7.0)0.48
Total10 969 (100.0)5826 (100.0)5143 (100.0)0.8848 917 (100.0)26 445 (100.0)22 472 (100.0)0.85

Procedures

The self-administered questionnaire included items concerning sociodemographic characteristics such as age, sex, and community size, and items concerning life stress. In addition, depressive symptoms were assessed using the Japanese version of the Center for Epidemiologic Studies Depression Scale (CES-D).16,17

The life stressors were assessed with the question: ‘What types of troubles, hardships, stress, or dissatisfaction with daily life did you experience during the past month? Please select all that apply’. The questionnaire was designed to ask participants to mark items only when they identified them as stressors, rather than simply asking about the presence of stressors. The list included a total of 21 choices subsumed under five domains: (i) problems with primary support group (separation/divorce; health/illness/care of self; death of a close person; burden of housework; family relationship; relationship with relatives; and health/illness/care of family); (ii) problems related to social environment (having no one to talk to; loss of purpose in life; having nothing to do; and retirement); (iii) occupational problems (commuting [crowded public transportation, long distance etc.]; workplace relationship; unemployment; adjusting to a new job; stress on the job); (iv) housing problems (relationship with neighbors; living environment [pollution, noise etc.]; concerns about housing); and (v) financial problems (debt; and income/household budget). The strength (burden) of life stressors was assessed with the question: ‘Have your troubles, hardships, stress, or dissatisfaction with daily life interfered with your social life or everyday life during the past month?’ Participants answered this question on a 4-point scale: 1, much; 2, some; 3, little; or 4, none.

The CES-D is a 20-item instrument specifically designed to screen for depression among the general population, and in the present study it was used to assess subjective depressive symptoms during the past week. Each item on the CES-D is scored from 0 to 3, yielding a total score ranging from 0 to 60, with higher scores indicating more severe depressive symptoms. A cut-off score of ≥16 may indicate the presence of depression.16 Almost 30% of Japanese adults reportedly score ≥16 on the CES-D,18 suggesting an overestimation of the prevalence of depression compared with Japan as a whole, as well as to Western European countries. Therefore, we defined a CES-D score of ≥26 as a cut-off to select subjects whose symptoms more closely approximate those of major depression according to the criterion used by Cho et al.19 As a result, the present study identified the following three groups of subjects based on the severity of depressive state: (i) control group scoring <16 on the CES-D (mean ± SD = 9.5 ± 4.0); (ii) D16 group having mild–moderate depressive symptoms with a score of 16–25 on the CES-D (mean ± SD = 19.8 ± 2.8); and (iii) D26 group having severe depressive symptoms with a score of ≥26 on the CES-D (mean ± SD = 32.8 ± 6.5).

Statistical analysis

After contrasting our study sample data with the Census data, we adjusted the sample size for gender and age group. The study sample was classified into two gender groups and four age groups in decades (Table 1). For each of these eight subgroups, we weighted the sample size based on the population proportion (weight(i) = percentage of Census population in subgroup(i)/percentage of sample in subgroup(i), where i = 1, . . . , 8).1 We conducted the following statistical analyses based on the weighted samples.

Mean CES-D scores were compared using two-way (age group × gender) analysis of variance (anova), followed by Bonferroni post-hoc comparisons. Differences in the distribution of subjects in the control, the D16, and the D26 groups as well as the male : female ratios were analyzed using the χ2 test.

We further examined the relationship between the incidence of life stressors and mild–moderate (D16) and severe (D26) depressive symptoms using multiple logistic regression analysis. The following parameters were entered as covariates: life stressors, gender, age group, community size (cities with population of ≥150 000 were coded as metropolis, while those with a population <150 000 were coded as town/village), geographic region (north, east, west, or south), and strength (burden) of life stressors. Odds ratios (OR) and 95% confidence intervals (CI) were calculated.

Statistical significance was set at 0.05. All analyses were performed using SPSS 11.5 for Windows (SPSS, Chicago, IL, USA).

RESULTS

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. ACKNOWLEDGMENTS
  7. REFERENCES

The questionnaire was returned by 32 729 participants. Because the number of residents in each sampling community who were contacted for participation in the ASHW was not made public by the Ministry of Health, Labour and Welfare, we were unable to calculate the response rate. The response rate for a similar survey, however, conducted 3 and 4 years previously, was 87.1% and 89.6%, respectively. We assume that the response rate for the present study, which used a similar methodology, resembled those. A total of 707 subjects who returned a blank questionnaire were excluded from the analysis. Further, subjects who did not respond to items on gender or age (n = 208) or those who did not respond to five or more items on the CES-D (n = 7471) were excluded from the analysis. Because the present study was focused on late life, we further excluded subjects who were younger than 50 years of age (n = 13 374). The final sample size was 10 969.

Figure 1 shows the mean CES-D scores by age group and sex. Two-way anova found a significant main effect of age group (F(3,10 961) = 82.3, P < 0.001). Post-hoc analyses indicated a significantly higher CES-D score among those in their 70s (P < 0.001) and 80s (P < 0.001) than those in their 50s and 60s, in other words, there were significantly more depressive symptoms with increased age. We also found a significant main effect of gender (F(1,10 961) = 18.5, P < 0.001). Women reported a significantly higher CES-D score than men (P < 0.001). Age group × gender interaction was not significant (F(3,10 961) = 1.3, P = 0.275).

image

Figure 1. Center for Epidemiologic Studies Depression Scale (CES-D) score vs age group and gender: (□) male; (bsl00001) female. Data are given as average ± SEM. Increased age was associated with higher CES-D scores. Compared with men, women scored significantly higher on the CES-D (*P < 0.001).

Download figure to PowerPoint

Table 2 lists the distribution of subjects in the control, the D16, and the D26 groups as well as the male : female ratio in each age group. Of the entire study sample, 2397 (21.9%) and 1019 (9.3%) fell under the D16 and the D26 groups, respectively. Across age groups, there were significant differences in the distribution of subjects in the three symptom groups (χ2 = 316.9, d.f. = 6, P < 0.001). The majority of the subjects in the D16 group were in their 80s, while those in the D26 group were older than 70. There were also significant differences in the male : female ratio across the three symptoms groups (χ2 = 40.9, d.f. = 2, P < 0.001). The male : female ratio in the D16 and the D26 groups was lower at 0.71 and 0.70, respectively, compared with 0.92 in the control group.

Table 2.  Age vs severity of depression
Age group%Control group (M/F ratio)%D16 group (M/F ratio)%D26 group (M/F ratio)
  1. Significant differences were found for both D16 and D26 with respect to gender and age (χ2 test, P < 0.001).

50–5971.5 (1.05)21.6 (0.85)6.9 (0.75)
60–6973.3 (0.97)20.2 (0.74)6.5 (1.02)
70–7965.5 (0.81)22.2 (0.63)12.3 (0.64)
≥8051.7 (0.50)27.4 (0.44)20.9 (0.47)
Whole68.9 (0.92)21.9 (0.71)9.3 (0.70)

Table 3 lists the percentage of subjects who endorsed the item as well as the relative risks (OR) for the presence of D16 and D26 symptoms for each of the life stressors, derived from and grouped based on the classification for the DSM-IV-TR Axis IV.20 The most frequently endorsed life stressors were ‘health/illness/care of self’ (34.2%), ‘health/illness/care of family’ (21.5%), ‘stress on the job’ (15.9%), and ‘income/household budget’ (15.7%).

Table 3.  Relative risk for the presence of D16 and D26 symptoms vs life stressor
Life stressor% respondersD16 groupD26 group
Adjusted OR95%CIPAdjusted OR95%CIP
  1. Covariates: life stressor, gender, age group, community size, geographic region, strength (burden) of life stressors.

  2. CI, confidence interval; OR, odds ratio.

Problems with primary support group       
 Separation/divorce0.72.81.4–5.30.0022.20.9–5.6NS
 Health/illness/care of self34.21.71.5–1.9<0.0012.21.9–2.7<0.001
 Death of a close person5.61.61.3–2.0<0.0011.51.1–2.00.006
 Burden of housework3.81.41.1–1.80.0061.71.2–2.40.004
 Family relationship12.51.51.3–1.8<0.0011.81.5–2.3<0.001
 Relationship with relatives8.21.51.3–1.8<0.0011.41.1–1.90.009
 Health/illness/care of family21.50.90.8–1.05NS0.80.6–0.90.005
Problems related to social environment       
 Having no one to talk to4.53.32.5–4.4<0.0015.03.6–6.9<0.001
 Loss of purpose in life6.41.81.5–2.2<0.0012.82.2–3.7<0.001
 Having nothing to do3.11.51.1–2.00.0162.41.7–3.4<0.001
 Retirement12.11.10.95–1.3NS0.80.6–1.1NS
Occupational problems       
 Commuting (crowded, long distance, etc.)0.91.50.9–2.3NS1.30.6–2.8NS
 Workplace relationship8.51.41.2–1.7<0.0011.51.1–2.00.014
 Unemployment1.91.30.9–1.8NS1.20.7–1.9NS
 Adjusting to a new job1.00.90.6–1.5NS1.10.6–2.1NS
 Stress on the job15.91.10.9–1.2NS0.80.6–0.90.030
Housing problems       
 Relationship with neighbors7.61.41.1–1.7<0.0011.61.2–2.0<0.001
 Living environment (pollution, noise, etc.)4.11.00.8–1.3NS1.00.7–1.5NS
 Concerns about housing6.10.90.7–1.1NS0.90.7–1.2NS
Economic Problems       
 Debt4.91.31.1–1.70.0142.11.5–2.9<0.001
 Income/household budget15.71.00.8–1.1NS1.00.8–1.2NS

Among problems regarding the primary support group, all items except ‘separation/divorce’ and ‘health/illness/care of family’ had significant relationships with the increased incidence of D16 and D26 symptoms. The relationship was especially strong for ‘health/illness/care of self’ (OR = 1.7 and 2.2 for D16 and D26 symptoms, respectively). ‘Separation/divorce’ had a strong relationship with the increased incidence of D16 symptoms (OR = 2.8), but its relationship with the incidence of D26 symptoms was not significant. In contrast, there was a significant relationship between ‘health/illness/care of family’ and decreased incidence of D26 symptoms (OR = 0.8).

Among problems related to social environment, ‘having no one to talk to’ (OR = 3.3 and 5.0), ‘loss of purpose in life’ (OR = 1.8 and 2.8), and ‘having nothing to do’ (OR = 1.5 and 2.4) had significant relationships with the increased incidence of D16 and D26 symptoms, respectively. Among occupational problems, only ‘workplace relationship’ had significant relationships with the increased incidence of D16 and D26 symptoms (OR = 1.4 and 1.5, respectively). ‘Stress on the job’ had a significant relationship with the decreased incidence of D26 symptoms (OR = 0.8). Among housing problems, only ‘relationship with neighbors’ had significant relationships with the increased incidence of D16 and D26 symptoms (OR = 1.4 and 1.6, respectively). Among financial problems, ‘debt’ had significant relationships with the increased incidence of D16 and D26 symptoms (OR = 1.3 and 2.1, respectively).

DISCUSSION

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. ACKNOWLEDGMENTS
  7. REFERENCES

The aim of the present study was to clarify the psychosocial stress in daily life associated with late-life depression. The study subjects were individuals aged ≥50 randomly selected throughout Japan. Their age distribution was comparable to that from the Census conducted around the same time. That is, the present study has epidemiological value due to its use of a large representative sample of the general population. This study included >10 000 subjects in late life who were living in 300 different communities across Japan, which enabled us to obtain data representing the Japanese general population. In the present sample approximately one in five (21.9%) and one in 10 (9.3%) subjects fell into the groups D16 (mild–moderate depressive symptoms with a score of 16–25 on the CES-D) and D26 (severe depressive symptoms with a score of ≥26 on the CES-D), respectively. Further, increased age and being female were associated with more severe depressive symptoms (i.e. higher scores on the CES-D). In order to examine whether the presence of subjective stress in late life is associated with the incidence of D16 and D26 depressive symptoms, survey questions were designed to ask participants to report stressors only when they identified items in the list as stress, rather than simply asking about the presence of stressors.

The study found the strongest relationship between incidence of both D16 and D26 symptoms and life stressors stemming from ‘having no one to talk to’. All other life stressors related to social relationships such as ‘relationship with neighbors’, ‘workplace relationship’, and ‘relationship with relatives’ were also significantly related to the presence of depressive symptoms. The association between diminished social contacts and the development of depression in late life has been established in previous studies.21,22 With the aging of the population, the number of Japanese elderly people living alone is markedly increasing. This is unlikely to be unrelated to the high prevalence of late-life depressive state found in the present study. Diminished social contacts in late life include attenuation of human relationships and insufficient social support. Indeed, previous research suggests that adequate social support not only directly improves psychological health, but may act as a buffer against social stress as a risk factor for depression.23 Therefore, improvement in nursing care insurance services in Japan, especially increasing service utilization among community-living elderly people may help combat late-life depression.

Next to ‘having no one to talk to’, experiences of loss and bereavement (‘loss of purpose in life’, ‘separation/divorce’, ‘death of a close person’, and ‘having nothing to do’) were strongly related to the incidence of late-life depressive symptoms. A number of studies in Europe and USA have consistently shown a strong relationship between the death of a spouse or a loved one and subsequent development of depression.24–27 Life events associated with a strong sense of loss that may be destructive to the individual and that may persist over a long period of time, such as separation from or bereavement of an important person, loss of purpose in life, and loss of social roles, have been identified as risk factors for late-life depression.15,28 The present results support the possibility that experiences of loss and bereavement may increase the risk for late-life depression among the Japanese as well. Previous studies that examined risk factors for depression did not identify ‘separation/divorce’ as a significant risk factor.29,30 This may be attributable in large part to insufficient statistical power to detect its effect due to the low frequency of occurrence. Although the proportion of respondents who selected ‘separation/divorce’ was also very low in the present study, at 0.7% (the least), a relationship was found between ‘separation/divorce’ and the incidence of D16 symptoms, due to the larger sample size. In contrast, among experiences of loss and bereavement, ‘separation/divorce’ was not significantly associated with the incidence of D26 symptoms. This finding suggests that even though ‘separation/divorce’ in late life was associated with mild depressive symptoms, examining whether this item could be a risk factor for moderate–severe clinical depression remains as a question for further study.

The third strongest relationship with late-life depressive symptoms was found for ‘health/illness/care of self’. This item was the most common life stressor, endorsed by 34.2% of the entire sample, 49.2% of the D16 group, and 66.4% of the D26 group. Studies on the elderly have repeatedly shown that having physical illness and/or disabilities increases the risk for developing depression.22,26,31,32 Cerebrovascular disease, in particular, is a risk factor consistently associated with the development of late-life depression.33 The influence of physical illnesses on the development of depression has primarily been attributed to biological processes, including alterations in the neuroendocrine system or cerebral blood flow and physical stress such as chronic pain. As a psychosocial risk factor, in contrast, physical illnesses play a role in one's psychological reactions when faced with aging or death or in social aspects such as hospitalization, institutionalization, and reduced social activities.15 Similar to other life events, development or exacerbation of a severe and fatal illness may incur strong psychological burden and frequent and significant confusion in lifestyle among afflicted elderly people. Some elderly people must face serious yet unavoidable issues such as their own senility, remaining days, or death upon receiving a diagnosis or being informed of serious or chronic physical illness such as cerebrovascular disease, cancer, myocardial infarction, or diabetes. These issues may result in impairment in life functioning or hospitalization, which in turn may diminish social contact.

Another life stressor significantly associated with late-life depressive symptoms was ‘debt’. It is generally well-recognized that economic status affects physical and mental health. It is therefore not surprising that the present study found a relationship between ‘debt’ and late-life depressive symptoms. Limited income leads to poor access to medical care and mental health services, which consequently hinder the early detection and treatment of depression. Even when depression is detected at a relatively early stage, financial hardship will hamper prevention of major depressive episodes or access to mental health resources ensuring appropriate treatment for the current depressive episode.34 Meanwhile, the present study did not find a significant relationship between ‘income/household budget’ and depressive symptoms. This may be attributable to Japan's universal health insurance system, in which people with low income have relatively easy access to medical care. Therefore, depression among the elderly people who have debts may be largely attributable to reduced quality of living conditions or psychological pain stemming from the obligation to repay the debt.

Finally, the relative risk of respondents who endorsed ‘health/illness/care of family’ and ‘stress on the job’ to have severe depressive symptoms (i.e. score ≥26 on the CES-D) was <1, suggesting that these two items were not identified as risk factors for clinical depression. These two items, however, were both endorsed at high frequencies overall, indicating that many of the subjects in the control (no depression) group also endorsed them. Therefore, the lack of relationship between increased incidence of depressive symptoms and either of ‘health/illness/care of family’ or ‘stress on the job’ observed in the present study does not guarantee that these items do not affect late-life depression.

Study limitations

There were several limitations to the present study. First, as a cross-sectional survey, it was not possible to ascertain the time of onset and duration of depressive symptoms and life stressors or the time interval between them. Therefore, a causal relationship cannot be inferred. Investigation into the causal relationship was outside of the scope of the present study, but is suggested for future research. We were able, however, to achieve the primary goal of the study, which was to clarify the relationship between life stressors and late-life depressive symptoms in a large representative sample of the general population.

Second, the survey data were collected via a self-administered questionnaire, and structured interview was not used to determine definitive diagnosis. Data collection using interview for a large sample in the present study would present tremendous methodological and financial challenges. Hence there is the possibility that some of the individuals defined as having depression in the present study may have had comorbid psychiatric disorders such as anxiety disorders.

Third, because the CES-D is a screening instrument for depression among the generations,16 some of the study subjects who scored 16 (the cut-off) or higher on the CES-D may not have met the clinical diagnostic criteria (e.g. DSM-IV-TR) for depression. The reliability and validity of the CES-D, however, have been widely established in epidemiological studies using a representative population sample. We therefore believe that the investigation into the relationship between life stressors and late-life depression is beneficial to gaining insight into how to combat the risk factors for depression.

Fourth, it is difficult to identify whether items included in the ‘problems related to social environment’ domain (such as ‘loss of purpose in life’ and ‘having nothing to do’) are stress factors or induced as a part of depressive symptoms. It is necessary to consider the possibility that the study results may include both.

Conclusion

The aim of the present study was to clarify the relationship between late-life depression and life stressors in a large representative sample of the Japanese general population. A relationship was found between late-life depression and diminished social relationships, experiences with loss of purpose in life or human relationships, and health problems. The findings provide valuable insights for policies to help sustain mental health in late life in rapidly ‘super-aging’ Japan, where the population is growing older at a rate incomparable to any other country.

ACKNOWLEDGMENTS

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. ACKNOWLEDGMENTS
  7. REFERENCES

This study was partly supported by a Health Science Research Grant from the Ministry of Health and Welfare, and by a Research Grant from the Japan Society for Promoting Science and Technology Agency. The authors report no other financial affiliation or relationship relevant to the subject of this article.

REFERENCES

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. ACKNOWLEDGMENTS
  7. REFERENCES
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